In response to certain wavelengths of electromagnetic radiation (or “actinic radiation”), photochromic compounds, such as indeno-fused naphthopyrans, typically undergo a transformation from one form or state to another form, with each form having a characteristic or distinguishable absorption spectrum associated therewith. Typically, upon exposure to actinic radiation, many photochromic compounds are transformed from a closed-form, which corresponds to an unactivated (or bleached, e.g., substantially colorless) state of the photochromic compound, to an open-form, which corresponds to an activated (or colored) state of the photochromic compound. In the absence of exposure to actinic radiation, such photochromic compounds are reversibly transformed from the activated (or colored) state, back to the unactivated (or bleached) state. Compositions and articles, such as eyewear lenses, that contain photochromic compounds or have photochromic compounds applied thereto (e.g., in form of a photochromic coating composition) typically display colorless (e.g., clear) and colored states that correspond to the colorless and colored states of the photochromic compounds contained therein or applied thereto.
Upon exposure to actinic radiation (e.g., sunlight), the photochromic compound typically is transformed from the unactivated (or bleached) state to the activated (or colored) state over a period of time that is referred to as an activation time. Correspondingly, when exposure to actinic radiation is halted (e.g., due to shielding of sunlight), the photochromic compound typically is transformed from the activated (or colored) state to the unactivated (or bleached) state over a period of time that is referred to as a fade time. It is generally desirable that the activation time and the fade time associated with a photochromic material in each case be minimized. In addition, it is desirable that the fade rate associated with a photochromic compound be substantially linear. With photochromic eyewear, such as photochromic lenses, a linear fade rate allows the wearer's eyes to adjust more smoothly and less noticeably to the wearer as the lenses transform from a colored to a bleached state.
Since photochromic compounds can be expensive, it is typically desirable to minimize the amount of photochromic compound or compounds used without compromising the photochromic properties, such as optical density, of the photochromic article with which the photochromic compounds are associated. With some applications, the photochromic compounds are present in a layer, such as a coating, that is applied over an underlying article, such as an optical lens, and/or the photochromic compound is present within the article itself, which can be achieved by methods such as imbibition and/or cast-in-place methods.
Photochromic compounds can be subject to migration within the matrix, such as an organic matrix, in which they reside. With, for example, a photochromic layer or coating, the photochromic compounds can migrate out of the layer, which can result in an undesirable decrease in the photochromic properties of the photochromic layer. In some cases, a photochromic compound can migrate from a relatively soft coating layer in which the photochromic compound has favorable properties, such as good fade kinetics, into an abutting coating layer that is relatively hard and in which the photochromic compound has less favorable properties, such as undesirable fade kinetics. The overall effect, in such cases, can be a photochromic article having undesirable photochromic properties, such as undesirable fade kinetics.
It would be desirable to develop photochromic compounds that are subject to reduced migration or substantially no migration within a matrix, such as an organic matrix, in which they reside. It would also be desirable that such newly developed photochromic compounds provide a desirable level of photochromic properties.